FIG. 1 illustrates a network structure of a universal mobile telecommunications system (UMTS).
The UMTS includes a user equipment (UE), a UMTS terrestrial radio access network (UTRAN), and a core network (CN). The UTRAN includes at least one radio network sub-systems (RNS), wherein each RNS includes a radio network controller (RNC) and at least one base station (Node B) managed by the RNC. At least one cell exists in one Node B.
A radio protocol used in the UMTS includes a plurality of layers, and a radio link control (RLC) of the layers serves to ensure quality of service (QoS) of each radio bearer (RB) and transmit data thereof. The RLC layer provides one or two independent RLC entities for each RB to ensure unique QoS of each RB, and also provides three RLC modes, i.e., a transparent mode (TM), an unacknowledged mode (UM), and an acknowledged mode (AM) to support various kinds of QoS.
The AM RLC is to ensure error-free data transmission through re-transmission. Accordingly, the AM RLC serves to mainly transmit unreal time packet data of a packet switching domain, such as TCP/IP, in a user plane. Also, in a control plane, the AM RLC serves to transmit RRC message which necessarily requires acknowledgement, among RRC messages transmitted to a specific user equipment within a cell.
A protocol data unit (PDU) used in the AM RLC can be divided into a data PDU and a control PDU. The control PDU can be subdivided into a status PDU, piggybacked status PDU, a reset PDU, and a reset Ack PDU.
Meanwhile, a media access channel (MAC) entity provided in a MAC layer determines how many data are transmitted by which RLC entity per transmission time interval (TTI). Each RLC entity configures RLC PDUs depending on the determined result of the MAC entity and forwards the RLC PDUs to the MAC entity.
In view of the RLC entity, if a lower entity is operated desirably, status information which will be exchanged between the respective RLC entities has an insignificant amount. For example, if a user equipment is located close to a base station, it is not likely that the RLC PDU forwarded to a lower end by the RNC entity fails to reach the other RLC entity as the RLC PDU is not transmitted well from the lower end. Accordingly, it is not likely that the RLC entity at a receiving side transmits status information to the RLC entity a transmitting side, wherein the status information is to request retransmission of a corresponding RLC PDU as the RLC entity at the receiving side fails to receive the RLC PDU.
However, in a mobile communication system, a user equipment changes its position continuously, and a communication environment may be changed rapidly even though the user equipment does not move. Also, if the user equipment is located at the boundary of one or more cells, or if the user equipment suddenly enters a tunnel, it is likely that data blocks transmitted from the user equipment and data blocks transmitted from the base station to the user equipment may be damaged in a radio interval. In this case, control information to be exchanged between the base station and the user equipment, i.e., status information increases. An example of the status information includes reception status information of data, i.e., predetermined status report, which is to report information as to whether each data block has been successfully received.
Meanwhile, in a status where a wireless environment is poor, the amount of data which the user equipment can transmit per transmission time interval (TTI) is limited. Accordingly, since the status report that can be transmitted from the RLC entity of the receiving side is limited to a certain size, the RLC entity of the receiving side should divide the status report into a plurality of status PDUs and transmit the divided status PDUs to the transmitting side. At this time, some of the divided status PDUs may not reach the RLC entity of the transmitting side for several reasons. As a result, the RLC entity of the transmitting side may wrongfully determine data reception status of the RLC entity of the receiving side.
For example, when one status report is divided into two status PDUs, it is supposed that the first status PDU reports that first to fifth RLC PDUs and seventh to tenth RLC PDUs have been successfully received and that the second status PDU reports that eleventh to fifteenth RLC PDUs have been successfully received.
If the RLC entity of the transmitting side successfully receives the second status PDU only, the RLC entity of the transmitting side may wrongfully determine that the RLC entity of the receiving side has successfully received the first to fifteenth RLC PDUs. Also, the RLC entity of the transmitting side may wrongfully determine that the RLC entity of the receiving side has not successfully received the first to tenth RLC PDUs. Moreover, if the RLC entity of the transmitting side receives the first status PDU after receiving the second status PDU, the RLC entity of the transmitting side may determine that among RLC PDUs generated after the fifteenth RLC PDU, the sixth RLC PDU has not been successfully forwarded. All the above problems may be caused as the large sized status report has not been successfully forwarded to the RLC entity of the transmitting side.